In 1932, the Type E coupler was adopted as the ARA, American Railway Association (predecessor to the AAR, Association of American Railroads) standard coupler for railway freight cars. Although modified periodically since then to meet changing requirements imposed by changing demands, and other coupler designs have been developed for special applications, the Type E coupler is today still the standard coupler for freight service. As is well known, the Type E coupler as well as other standard use couplers, have a degree of free and cushioned slack. That is, a certain amount of free "play" exists between the coupler components when the load is changed from draft to buff loading, and visa versa. At the same time, the draft gear acts as a spring mechanism to cushion impact between adjacent cars. It has been found that eliminating the free and cushioned slack within a train can eliminate over the road train action forces due to "run-ins" and "run-outs". The magnitude of these forces are large and cause significant wear and tear of the rolling stock, and in some cases can be significant enough to cause derailments.
More recently, slackless drawbar coupler have come into use which were developed for use in unit train applications where interconnected cars are uncoupled only rarely for periodic inspection and repair, with the coupling essentially comprising a rigid drawbar with one end pivotally connected to one car and the other end pivotally connected to the adjacent car. Such jointed cars are not subjected daily to impact forces associated with bumping encountered in classification yards, and, therefore, do not require cushioning devices such as draft gears. Accordingly, because of their significant lighter weight, such slackless drawbar couplers are in widespread use in unit trains, such as coal trains, and other captive use applications.
An example of such a slackless drawbar coupling is disclosed in U.S. Pat. No. 4,580,686, the disclosure of which is incorporated herein by reference. This patented coupling system provides a drawbar arrangement for coupling railway cars each having a center sill and trucks at its opposite ends, the trucks being pivotal about vertical king pins. The drawbar has an enlarged spherical butt end portion defining essentially convex spherical buff and draft load surfaces, a rear support block having a tapered rear surface and a concave substantially hemispherical buff load bearing surface adapted to engage with the convex buff load bearing surface of the butt end portion of the drawbar, a slack adjusting wedge for engaging the tapered surface of the rear support block, means for transferring buff loads from the slack adjusting wedge to the center sill, a front draft block having a concave and substantially hemispherical draft load bearing surface adapted to engage with the convex draft load surface of the enlarged spherical butt end portion, the front draft block including an annular draft load surface opposite the hemispherical draft load surface thereof, a wear block having an annular draft load surface adapted to engage the annular draft load surface of the front draft block, and means supported by the center sill for transferring a draft load from the wear block to the center sill. Although there are other slackless drawbar designs, most can be divided into two basic types, those in which the drawbar is rotary, as described above where the drawbar has a spherical head portion, and those where the drawbar is not rotary, as for example, where the end of the drawbar is secured with a single pivot pin securing it to a base structure.